Where do They Occur?

There is a possibility for a tornado to form anywhere where the atmospheric conditions are favourable. There is a lot of information and publicity about tornadoes in the USA and there is no doubt that some of those are very dangerous and cause considerable damage and loss of life. However, when area is taken into account, they are more frequent over the UK. The main difference is that very few British tornadoes cause significant damage or loss of life.

The area known as the Mid-West, between the Rockies in the west and the Appalachians in the east, has been termed 'Tornado Alley'. This is where the greatest number of the most damaging tornadoes occur and there is a good reason why this should be the case. The area around the Gulf of Mexico is hot and humid. At the same time, the air that passes over the Rockies can be cold and, perhaps more importantly, is very dry. It is this combination that can lead to the development of enormous cumulonimbus clouds growing up to nearly 18,000 metres (60,000 feet). Occasionally, the clouds may group together into what is known as 'supercells', and it is from these that the most dangerous tornadoes are likely to form - along with intense thunderstorms and damaging hail showers. The cumulonimbus clouds that develop over the UK rarely extend above 12,000 m (40,000 ft) and are consequently less 'energetic' than those over America.

The only continent where tornadoes have not been reported is Antarctica. They have occurred in every US state, including Alaska, though very rarely west of the Rockies. Tornadoes often occur in groups, known as 'outbreaks', and the greatest number on any one occasion was during the Super Outbreak on 3-4 April 1974 when 148 were reported from 11 states. As a comparison, there were 105 tornadoes reported in England within a few hours on 21 November 1981.